Technology Status of Hydrogen Road Vehicles
Technology Status of Hydrogen Road Vehicles
Technology Status of Hydrogen Road Vehicles
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high compression ratios (higher power and efficiencies) can be exploited. Therein lies the challenge because<br />
only LH 2 can be considered (a GH 2 storage system would need to carry a permanent ballast <strong>of</strong> 100 bar<br />
unuseable gas); mixing must be accomplished in very few milliseconds; and cryogenic pumps and injectors<br />
when cavitation is always incipient, and the fluid has no lubricating potential, simply do not exist except in<br />
experimental versions.<br />
So some projects are starting with low-pressure IMF, where the H 2 is injected after the inlet valve closes but<br />
before the pressure rises above 10 bar. The injector technology is not too demanding, mixing has more time<br />
to be complete, high compression ratios can be used and, with turbocharging, the power output can be<br />
maximized; in fact, power output can approach 120% <strong>of</strong> the gasoline engine if combustion is smooth.<br />
Unfortunately, early (low-pressure) IMF is subject to pre-ignition for reasons mentioned earlier and which<br />
Japanese researchers blame on the unavoidable hot combustion products present in the cylinder on the inlet<br />
stroke. According to Furahama (1989), this pre-ignition can cancel the 20% theoretical power gain.<br />
Furahama et al. (1991) give details on combustion processes for the Musashi-8 small sports car with a<br />
modified four-cylinder Nissan diesel engine illustrated as item 14 in Table 1. This car, the latest in a series<br />
<strong>of</strong> H 2 test vehicles from this leading Japanese institute, was equipped with updated high-pressure LH 2 pump<br />
and injector, and relative control: LH 2 pump in the cryogenic tank and powered by a direct current (DC) motor<br />
with accurate control <strong>of</strong> speed and pressure; high compression ratio (13.5), and hence higher LH 2 pressure (10<br />
MPa);separate spark control on each cylinder; injector nozzle with eight holes and actuated by oil pressure;<br />
and a combustion cavity in the crown <strong>of</strong> each piston.<br />
The combustion details were first investigated and adjusted on a single-cylinder optical research engine,<br />
similar to a single cylinder <strong>of</strong> the four-cylinder Nissan engine. Above all, the very crucial values <strong>of</strong> injection<br />
timing and ignition timing--absolute and relative values--were fine-tuned. The combustion processes were<br />
visualized through the piston, with mirrors installed on the cylinder head, and on the intake and exhaust valves.<br />
High-speed laser-enhanced Schlieren photographs showed density differences in the mixture as it combusted.<br />
In addition to the wealth <strong>of</strong> overall data, it was found that if the eight hydrogen jets are ignited when the<br />
nearest tip reaches the spark, combustion spreads smoothly one after the other to the remaining jets, and the<br />
cylinder pressure rise is optimized; delaying the spark by only 0.4 ms gave poor combustion. In both cases<br />
injection started at 181 BTDC, and spark occurred at 151 (good combustion) or 141 (poor combustion) BTDC.<br />
Applied to the main engine running at higher speed, since the time for the hydrogen jet to travel from injection<br />
to spark is constant, a variable but highly precise spark timing allowed good combustion in road tests up to<br />
130 km/h.<br />
These results were then used in a study with two other engines to measure the NO x emissions, which can in<br />
principle be similar in any air-breathing ICE (Ninomiya et al. 1992). The formation <strong>of</strong> NO x depends<br />
essentially on the flame temperature and duration at high temperature; cooling the mixture and/or retarding<br />
the injection timing (for diesels) are well-known methods <strong>of</strong> lowering flame temperature/duration. Another<br />
way, applicable only in H 2 ICEs, is to recycle the cooled exhaust (essentially air and H 2O) so that the heat for<br />
water evaporation cools the mixture. The report gives the details <strong>of</strong> the tests on H 2 ICEs and the results<br />
obtained with all three methods, singly and in combination. Ten percent exhaust gas recirculation is sufficient<br />
to more than halve the NO x and to meet the emission standards in force, without any effect on attainable<br />
power; a similar result is obtained with cryogenic (-120°C) compared to ambient H 2; a large synergistic effect<br />
comes from the combination <strong>of</strong> exhaust gas recirculation (EGR) and cryogenic H 2; ignition retardation seems<br />
overall to have less effect.<br />
These Japanese studies continued with another one by the same team (Koyanagi et al. 1993), taking up<br />
surprisingly the external mixture formation in combination with internal direct-injection <strong>of</strong> LH 2 at higher<br />
powers. This is expected to overcome the need for delicate adjustment <strong>of</strong> injection and ignition timing noted<br />
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